The present invention relates to linearized Mach Zehnder amplitude modulators and interferometers and, more particularly, to an improved linearized Mach Zehnder amplitude modulator wherein the operating frequency thereof is extended into the tens of GHz range.
In the reference Betts et al., xe2x80x9cSub-Octave-Bandwidth Analog Link using Linearized Reflective Modulator,xe2x80x9d Fifth Annual ARPA Symposium on Photonic Systems, January 1995, there is described a linearized Mach Zehnder interferometer that operates in a double pass, reflection mode. As stated in that reference, optical analog links for antenna remoting applications often require both a large linear dynamic range and a low noise figure. Most of such links need less than one octave bandwidth because antennas are bandpass devices. The Betts et al interferometer basically comprises two standard Mach Zehnder interferometric modulators connected in series and optimized for exactly these criteria. A schematic diagram of a preferred implementation thereof is shown in FIG. 1.
Referring to FIG. 1, an optical analog link is shown which includes a laser 10, an optical circulator 12, a detector 14 connected to an RF output terminal 16, and a linearized modulator device 18. The light output is obtained through the optical circulator 12 (at terminal 3) and passes to detector 14. The modulator device 18 represents a simplification of the basic modulator described above wherein, in essence, the two modulator construction is cut in the middle, one of the modulators is discarded, and a mirror is added. As illustrated, the result is a single modulator 22 in combination with a mirror 20 disposed so that light passes through the modulator 22 twice, effectively duplicating the two modulator construction, while requiring only a single RF drive. In FIG. 1, the RF drive input and bias input are indicated at terminal 24 which is connected to the modulator electrode structure, indicated schematically by an electrode 26 formed by a metal strip. The complete electrode structure also comprises a ground plane formed by a second metal strip (not shown). In the Betts et al reference, the modulator bias point provided at terminal 24 is 104.5xc2x0, rather than the conventional 90xc2x0 or quadrature point, in order to provide minimum third-order distortion. In this regard, it will be appreciated that because the Betts et al reference is concerned with single octave linearity, second-order distortion, while present, is out of band and is thus ignored.
In summary, in the linearized modulator of the Betts et al reference, the optical signal is reflected upon itself by mirror 20 so as to traverse the interferometer 22 in the opposite direction. By adjusting the bias voltage applied to input 24 at a particular point (104.5 degrees), linearized operation is achieved. This can be contrasted with the operation of a conventional single pass interferometer operated at the 90 degree or quadrature point. Such a 90 degree bias point would not provide linearized operation in the modulator of FIG. 1.
A disadvantage of the interferometer described in the Betts et al reference is that the device is essentially limited to operation at a few hundred MHz. This limitation on the operation can be traced to the electrode structure used, i.e., the use of resistive-capacitive (RC) electrodes. With an RC limited electrode structure, above a few hundred MHz the electrical phase changes while the optical signal or beam travels beyond the electrode structure 26 to the mirror 20. This is a serious limitation in that some important applications (e.g., cellular telephones) require such a device to operate in the bands 800-1000 MHz and 1800-2200 MHz.
In accordance with the invention, a traveling wave, linearized, reflection modulator is provided which can operate in the tens of GHz range and up to about 100 GHz with proper design. At low frequencies (i.e., less than 500 MHz) the drive voltage required should also be reduced as compared with the prior art RC limited electrical structure described above, because the reflected field will, as explained below, double the total field until propagation effects become important. As indicated previously, in the prior art RC limited electrode structure discussed above, electrical phase error limits the linearization beyond a few hundred MHz because the electrical phase changes while the optical beam travels beyond the electrode structure to the reflecting mirror. As will appear, this effect does not occur in the traveling wave implementation provided in accordance with the present invention because of the electrical-optical match provided by the invention and because the electrical and optical pathlengths are essentially the same.
In accordance with the invention, a traveling wave, linearized, modulator device is provided which comprises: an electro-optical modulator comprising a substrate; an optical waveguide disposed on said substrate and comprising an input-output portion, a terminal portion having a distal end and a plurality of arms disposed between, and connected to, the input-output portion and terminal portion, the arms being offset with respect to the input-output portion and the terminal portion and extending parallel to each other, and at least a first one of the arms having an optical characteristic that varies responsive to supplying of an electric field thereto; a reflective means, located at the distal end of the terminal portion of the optical waveguide, for causing light entering the input-output portion and propagating through said optical waveguide to be reflected back through the optical waveguide so as to exit through the input-output portion; and a traveling wave electrode structure disposed on the substrate and terminating in an open circuit substantially at the distal end of the terminal portion of the optical waveguide, the electrode structure including an electrode disposed on the first arm of the plurality of optical waveguide arms so as to supply an electric field to the first arm responsive to an electrical signal being supplied to the electrode; and means for supplying a radio frequency electrical signal and a direct current bias to the first electrode such that the radio frequency signal is biased at a non-quadrature bias point providing minimum third-order distortion, thereby providing substantially linearized modulator operation.
Advantageously, the bias point is set between 100xc2x0 and 110xc2x0. Preferably, the bias point is set at 104.5xc2x0.
Preferably, the electrode structure further includes a second electrode disposed on a second arm of said plurality of arms. Advantageously, the electrode structure further includes a third electrode disposed in an area of the substrate spaced from the plurality of arms so as to not overlay or be disposed on any of the arms. The second and third electrodes are preferably connected to ground. The reflective means preferably comprises a reflective coating disposed at the distal end of the terminal portion of the optical waveguide.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.